1. Field of the Invention
This invention pertains to borehole drilling apparatus and more specifically to that part of a drill string known in the industry as a stabilizer.
2. Description of the Prior Act
Stabilizers, sometimes referred to as drill collar stabilizers or as drill stem stabilizers, have been employed in earth boring operations for the petroleum industry to centralize the drill stem in the borehole, usually especially in the drill collar section at a distance of from 100 feet to 300 feet above the drill bit. The purposes of a stabilizer are to (1) help control hole angle direction, (2) prevent the bit from drifting laterally, which would result in undesirable dog-legs and ledges, and (3) improve bit performance by forcing the bit to centrally rotate about its axis so as to provide substantially equal force loading on all three drill bit cones. In addition, stabilizers also may be used to provide a reaming function for undersized or irregularly shaped boreholes provided the formation is not too hard. Rolling cutter reamers are employed to provide these functions for formations too hard to be thus treated by a stabilizer.
Stabilizers may be further categorized as rotating stabilizers and as non-rotating stabilizers. Non-rotating stabilizers do not rotate as the drill string is turned, its wall-contacting members merely moving longitudinally along the wall as the drill string is lowered and raised. On the other hand, a rotating stabilizer includes wall-contacting members that rotationally track along the wall of the borehole as the drill string is turned. In addition, rotating stabilizers can be further divided into fixed-contact and rolling contact types, which latter type would be functionally equivalent to a roller-reamer.
The contacting members of a fixed-contact type of stabilizer, which is the type of stabilizer described herein, are subjected to the various forces attendant to the entire drill string, the severest of which is often the longitudinal thrust force and the most constant and aggravating of which are the fretting forces. It should be noted that forces applied to the drill string are a result of the drill string manipulations, the conditions of the bore, and the fluid conditions internal and external to the drill string.
One fixed-contact type stabilizer is shown in U.S. Pat. No. 3,454,308, Kennedy, in which two wear bars are positioned end-to-end on either side of a locking bar within an accommodated slot, the wear bars being dovetailed to fit under tapered end surfaces of the slot and a locking bar. The locking bar is secured by a cap screw. Should the cap screw not be adequately tightened or should the cap screw vibrate loose, the wear bars are in danger of falling out during use of the tool.
Another fixed-contact type stabilizer is shown in U.S. Pat. No. 3,818,999, Garrett, in which the wear elements are accommodated in a V-section groove, the wear elements being held in place by cap screws. Only the cap screws provide surfaces for resisting the thrust forces and a broken or loosened cap screw will cause the wear element ordinarily held thereby to be dislodged.
A third type of fixed-contact stabilizer is shown in U.S. Pat. No. 4,106,823, Bassinger, in which tapered pairs of wear pads are wedged side by side in an accommodating slot, the pads being dovetailed along their sides and held thereby by tapered slot side surfaces to secure against lateral dislodging. Such pads are set in place by striking the ends with a mallet or hammer, with variable results depending on how well the tapered surfaces fit together and on the human element. High thrust loading and fretting can loosen and dislodge such pads during use either because the pads are not tightly seated or because the uneven surfaces do not permit uniform tightening along their entire lengths.
The history of fixed blade stabilizers leading up to the present invention may be approximated as follows:
Back in the 1950's stabilizer blades were welded on to the stabilizer body. When replacement was necessary, the blades were cut off with a torch and new ones were welded on. This construction is suitable for a wide range of sizes, e.g., 6 inches to 26 inches in diameter.
Later, it was decided that on the smaller sizes, e.g., 61/8" to 77/8", it was less expensive to form the stabilizer with integral blades and throw away the whole stabilizer when it was worn out. For lager sizes, e.g., diameters 77/8" to 171/2", it became the practice to form the blades so as to be integral with a sleeve which was shrunk fitted to the body. (For smaller sizes, exterior flow passage area would be too small to employ the shrink-sleeve construction.)
For those stabilizers where the shrink-sleeve construction was employed, the shrink-sleeve could be replaced when the blades were worn out. The old sleeve would be cut off with a torch, but heating of a new sleeve to be substituted required shop equipment. For field replacability, a construction in which the sleeve was screwed onto the body was developed, as shown in U.S. Pat. No. 3,754,609, Garrett.
For a similar range of sizes, e.g., 77/8" to 177/8", a less expensive stabilizer employing a body with integral blades and bolted on replaceable wear pads (studded with carbide inserts or hard faced) on the ends of the blades were introduced. (See, for example, U.S. Pat. No. 3,680,064, Crews, et al.; cf., U.S. Pat. No. 3,818,999, Garrett). Difficulty was experienced with occassional loss of the retention screws. See, for example, U.S. Pat. No. 4,280,742, Justman. In the larger sizes, excessive wear on the stabilizer nose (the tapered lower surfaces of the blades) was experienced. To protect against nose wear, blocks were welded into the lower tapered faces of the blades, (as is employed in the commerical product line of Drilco Division of Smith International, Inc.).
A proposal was made by the present inventor to provide a stabilizer having a body with integral blades and to drive fit into the blades round stepped wear pads (studded with carbide inserts or hard faced) with greater interference on the outer larger diameter cylindrical lands of the pads than on the inner smaller diameter cylindrical lands. These fits were similar to the fits for the stepped, round, shaft-supporting blocks employed at applicant's direction circa 1978-1979 by applicant's assignee in its roller stabilizers embodying the invention of applicant's U.S. Pat. 4,182,425, Garrett.
This proposal offered a structure that would be suitable for large diameter stabilizers, e.g., 14 inches or more in diameter, especially when employed with skewed blades, but would present problems due to insufficient exterior flow passages in the smaller sizes. This proposed stabilizer has not yet been built.
The present application construction provides a replaceable wear pad construction suitable for the full range of sizes from about 61/8 inches up to 26 inches and employs rectangular wear blade components (studded with carbide inserts or hard faced) with interference fits along the paraxial or lateral edges. A special interference fit is employed, as set forth hereinafter and delineated further below.
Therefore, it is a feature of the present invention to provide an improved stabilizer in which the wear elements are tightly secured in their respective accommodating pockets at least partially by an interference fit so as to minimize wear caused by movement of the elements within such pockets.
It is another feature of the present invention to provide an improved stabilizer in which the wear elements are installed by a lip-and-groove connection, which permits end-to-end close fitting of such elements in a common pocket, and which still permits ready removal for replacement purposes.
It is still another feature of the present invention to provide an improved stabilizer in which the wear elements are held, in part, by a block having connecting interlocking parts that secure the elements from becoming dislodged because of heavy longitudinal thrust loading.